The present invention is in the field of optical waveguide manufacture and relates particularly to apparatus for analyzing the optical characteristics of transparent, cylindrically symmetric articles such as optical waveguide preforms.
Many of the processes presently used to fabricate glass optical waveguide fibers involve the preparation of a cylindrical glass preform which is inspected before it is drawn into the final waveguide filament. The refractive index distribution within this preform ordinarily dictates the refractive index distribution obtainable in the drawn filament, which in turn controls the propagation characteristics of the waveguide. The desirability of examining the preform to determine whether it will provide an appropriate refractive index profile in filament form has thus been recognized, and several techniques for determining this refractive index distribution have been proposed.
P. L. Chu, Electronics Letters, 13 (24), pp, 736-738 (1977), has noted that the refractive index profile of an optical waveguide preform can be deduced from the exit trajectories of a series of parallel light rays entering the preform at right angles to its axis and back-scattered therefrom. D. Marcuse, Applied Optics, 18 (1), pp. 9-13 (1979), has shown that the refractive index distribution in a fiber or preform can be determined by observing the power distribution in a light field behind a preform or fiber front-illuminated by a broad, collimated light beam. I. Sasaki et al., Electronics Letters, 16 (6), pp. 220-221 (1980), have described a technique wherein spatial filtering of the light traversing a front-illuminated preform yields a shadow image of the deflection function, but the technique relies on a traversing diode array. L. S. Watkins, Applied Optics, 18(13), pp. 2214-2222, describes a procedure for computing the profile coefficient .alpha. and the refractive index difference .DELTA. between the center and outer core, but does not directly calculate the index as a function of preform radius.
The refractive index profile of an optical waveguide preform is commonly defined by a function correlating the refractive index of the preform at a given point with the radial distance of the point from the central axis of the preform. In Applied Optics, supra, Marcuse reports an expression for calculating the refractive index n(r) of a preform at radius r from the preform axis, based on the exit trajectories of parallel incoming light rays traversing the preform and refracted thereby, as follows: ##EQU1## wherein: n(r)=the refractive index of the preform at radius r
n.sub.c =the refractive index of the outer layer of the preform PA0 t=the entrance height of an incoming ray PA0 a=the radius of the preform PA0 L=the distance from the preform to an observation plane located behind the preform PA0 y(t)=the height, in the observation plane, of the exiting refracted ray having incoming height t
The height of the entering ray and the height of the refracted ray at the observation plane refer to the spacings of these rays from an optical axis parallel to the incoming rays and intercepting the axis of the waveguide preform.
In the Marcuse method, t and y(t) are not obtained directly but must be calculated from the power distribution of refracted light at the observation plane by an integration, with n(r) then being calculated by a second integration. This approach somewhat reduces the sensitivity of the method to fine detail in the refractive index profile of the optical waveguide preform.